Spectrum analyzers commonly contain a master or reference oscillator that generates a precise oscillation frequency to control the timing and measurement frequency of the analyzer. As such, the performance of spectrum analyzers may be significantly degraded by frequency errors in the reference oscillator. Typically, the reference oscillator is a crystal oscillator that provides a relatively stable oscillation frequency. However, crystal oscillators often exhibit thermal variation in the oscillation frequency. As a result, the oscillation frequency of a crystal oscillator may vary with ambient temperature. The amount of thermal instability is typically measured in parts per million and is used to specify the frequency precision of the crystal oscillator. From the frequency precision, the frequency error expected from the crystal oscillator can be determined.
As an example, a crystal oscillator with a nominal operating frequency of 10 MHz with a frequency precision of 1 part per million (ppm) has an expected frequency error of ±1 kHz. Although this frequency error is small in comparison to the nominal operating frequency, the error may be unacceptable in frequency sensitive applications. For example, when measuring narrowband signals using local oscillators whose frequencies are derived from the reference oscillator, a 1 kHz frequency error in the reference oscillator can significantly reduce the measurement accuracy of the spectrum analyzer.
There are a number of techniques currently available to correct frequency errors in the reference oscillator. One common technique is to lock the reference oscillator to a more accurate signal at the desired operating frequency (e.g., an externally-generated 10 MHz reference clock signal) to correct for any variations in the oscillation frequency. For example, the reference oscillator can be “locked” to the external signal by comparing the frequency of the reference oscillator to the frequency of the external signal to determine a frequency error or offset which is used to correct the reference oscillator frequency.
However, such automatic frequency correction (AFC) techniques require the reference oscillator to be adjustable (e.g., a voltage controlled oscillator), which increases the cost of the reference oscillator. Thus, in spectrum analyzers with inexpensive, fixed-frequency reference oscillators, frequency correction using available AFC techniques is not possible. In addition, spectrum analyzers may not have access to an external signal, and therefore, may not be able to correct the oscillation frequency of the reference oscillator. As a result, there is a need for a spectrum analyzer capable of correcting frequency errors in the reference oscillator without requiring the reference oscillator to lock to an accurate signal.